The present application relates to recovery of valuable tantalum from capacitor anodes and other product forms of tantalum mixed with waxy or oily materials, such as stearic acid binders of pressed tantalum pellets.
Tantalum capacitor manufacturers frequently add a binder to tantalum powder prior to pressing pellets. The binder, of which stearic acid is a typical example, helps to lubricate the press die, improves the green strength of the pellet and can contribute to more open porosity and higher capacitance of the sintered pellets. The pressed pellets are heated to 300.degree.-500.degree. C. in vacuum or in an inert gas atmosphere to remove the binder. This debindering operation, even when carried out with great care, is unable to completely remove all residues of carbon associated with the binder. The pellets contain 50-150 ppm carbon, which has been shown to degrade the electrical quality of the anodic oxide film electrochemically deposited on the surface of the sintered pellet. Until recently, most of the residual binder carbon was removed from the pellets during sintering at temperatures of 1,600.degree. C. and above. The new generation of very high CV capacitor grade tantalum powders are frequently sintered below 1,600.degree. C. to take full advantage of the capacitance capability of these high surface area powders. At these lower sintering temperatures, little, if any, clean-up of the tantalum surface occurs, and the carbon deposited on the surface of the sintered pellet will cause the quality of the anodic oxide film to decrease.
Another consequence of removing the binder from pressed pellets by vacuum or inert gas heating is an increase in the oxygen content of the pellets. Heating even at temperatures in the 300.degree.-500.degree. range coupled with the reducing tendency of the binder causes oxygen to pass into the tantalum substrate and strips the tantalum surface of the existing thermal oxide layer. Subsequent exposure of the tantalum surface to oxygen leads to the formation of a new thermal oxide layer and an attendant increase in the oxygen content of the tantalum of 500-1000 ppm. It is well known that a high oxygen concentration will degrade the electrical quality of the anodic oxide film on tantalum. The 500-1000 ppm of oxygen introduced into the tantalum during the binder removal process can be sufficient to cause failure of capacitors, especially if they are made from pellets which are double sintered. To date, capacitor manufacturers have not been able to successfully process double sintered pellets made from powder above the 23,000 CV/gram class if a binder removal step was included in the pellet preparation process.
It is an object of the invention to provide a way of removing binder from pressed pellets that will not add deleterious amounts of carbon and oxygen to tantalum pellets and will therefore not add the performance failures associated with such unwanted carbon and oxygen addition.
It is a further object of the invention to make tantalum pellets more tolerant to oxygen pick up at the sintering stages of processing.
It is a further object of the invention to provide a new approach to scrap tantalum recovery (recovery of valuable tantalum from used electrolytic capacitors) that is more suitable for recycled use compared to conventionally recovered tantalum.